The present invention relates to a display apparatus, a display system, and a display control method.
An information processing system (or apparatus) normally use s a display apparatus a s a means for realizing a visual expression function of information. As is well known, a CRT display apparatus is popularly used as such display apparatus.
Since a CRT display apparatus itself has no display memory function, display data must be supplied to the display apparatus at all times, and when the supply of display data is stopped, the display operation on the display screen is immediately interrupted.
For this reason, the display control for the CRT display apparatus must constantly execute an image write operation and a display image read-out operation to and from a video memory (to be referred to as a VRAM hereinafter) arranged in the information processing apparatus.
In the case of the above-mentioned CRT display control, since the operation for writing display data in the VRAM to update display information and the operation for reading out data from the VRAM to attain a display operation are independently performed, a program on the information processing system side can write desired display data at an arbitrary timing regardless of the display timing.
However, in general, since a CRT display apparatus has a larger depth in proportion to its display area, the volume of the entire CRT display apparatus increases. That is, the CRT display apparatus suffers a limited installation space, poor portability, and the like, and cannot attain a size reduction.
A conventional display apparatus has a function of merely receiving and displaying display information, and exchanges only a signal line for informing the ready state of the display apparatus, a signal line for transferring a reception clock signal for determining the display data reception timing, and the like as the information contents of the display information to the host side. Therefore, the actual state of the display apparatus side cannot be detected by the host side.
Furthermore, along with the recent advances in the semiconductor techniques, most of display apparatuses are controlled by one-chip CPUs or special-purpose control chips. The display apparatus is controlled in accordance with the control procedures pre-stored in its internal ROM. The control procedures are individually created in accordance with the specifications of the host side, and variable parameters and the like are set using hardware switches or setting data in a RAM.
More specifically, the conventional display apparatus is manufactured independently of the host side, and must have incompatible special-purpose control procedures in correspondence with the number of types of specifications of the host side. It is impossible to change the specifications written in the ROM, and such change can only be attained by exchanging display apparatuses or corresponding hardware components.
The present invention has been made in consideration of the above-mentioned problems and has as its object to allow an information supply apparatus side to cope with every state of the display apparatus since the information supply apparatus side can access the contents of a storage means of the display apparatus in practice.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.